From seven to three: Integrative species delimitation supports major reduction in species number in Rhodiola section Trifida (Crassulaceae) on the Qinghai-Tibetan Plateau
An essential component of biodiversity research and conservation is to accurately delimit species. Traditional morphology-based taxonomy has faced great challenges in complex taxa. In the last few years, the focus of species delimitations has shifted from defining species using different species concepts to integrating multiple lines of evidence to characterize different aspects of the speciation process. This study employs an integrative taxonomic method to revise the species limits in Rhodiola sect. Trifida, based on multi-locus sequence data from population-level sampling, morphometric analyses, and ecological niche modeling. We surveyed 44 populations representing seven previously defined morphological species. We sequenced three plastid markers and three nuclear markers, and employed the coalescent-based species delimitation method to assess the variational patterns from the molecular data. We also conducted a comprehensive morphological analysis, as well as ecological niche modeling for each species. The multidisciplinary data support Rhodiola liciae and R. sinuata as distinct species, which likely originated by allopatric isolation and ecological speciation. Rhodiola ovatisepala, R. sacra and R. sexifolia are suggested to be synonyms of R. chrysanthemifolia, and the status of R. alterna needs further verification. Our study suggests that the previous taxonomy of this group may depend on too many intra-lineage variable traits, and showcases the importance of integrative taxonomy for species delimitations in morphologically complex plant groups.
